The present disclosure relates to a fixing device for fixing a toner image on a sheet, and an image forming apparatus including the fixing device.
A fixing device of an image forming apparatus includes a fixing nip defined by a fixing roller and a pressing roller brought in pressed contact with each other. A sheet passes through the fixing nip where the sheet is pressed and heated to thereby allow a toner image on the sheet to be fixed thereon. There is known a fixing device including a pressure adjustment mechanism for adjusting a pressure contact force (nip pressure) in the fixing nip. The pressure adjustment mechanism makes it possible to set the nip pressure to an appropriate value according to the thickness of a sheet and, when the fixing device is not used, to release the nip pressure to thereby prevent distortion of the fixing roller due to its pressed contact.
Such a fixing device as described above needs to include a drive system for the fixing roller and a drive system for the pressure adjustment mechanism, the two drive systems being controllable independently of each other. Each of the drive systems is preferred to include a drive motor. However, in many cases, a single drive motor is used to drive the both drive systems for the purpose of reducing the number of components. In this case, the single drive motor rotates a driving input gear which is common to the both drive systems in a forward direction or a reverse direction, so that one of the drive system for the fixing roller and the drive system for the pressure adjustment mechanism is driven. Such a configuration requires a mechanism for transmitting only a torque of a specific direction selectively to one of the drive systems. In the above-described fixing device, a one-way clutch is used.
The one-way clutch is functionally satisfactory but is relatively expensive and is, therefore, desirably avoided for use. As an alternative to the one-way clutch, there are a ratchet mechanism and a planetary gear mechanism.